This invention relates to a device for detecting concentrations of oxygen in gases, the device being of the type having an oxygen sensor element of the concentration cell type using an oxygen ion conductive solid electrolyte and having a heater, a power supply circuit to apply a controlled voltage to the heater and a current supply circuit to supply a controlled current to the sensor element in order to maintain a reference oxygen partial pressure in the sensor element.
Recently oxygen sensors have been largely applied to automobiles for the purpose of detecting concentrations of oxygen in exhaust gases discharged from the automotive engines as the basis for electronic feedback control of air/fuel ratio of the gas mixture supplied to the engines. Usually oxygen sensors for this purpose are of the concentration cell type having a layer of an oxygen ion conductive solid electrolyte such as zirconia containing a small amount of a stabilizing oxide. In an oxygen sensor of this type is is necessary to maintain a reference partial pressure of oxygen on one side of the solid electrolyte layer.
In this field, a recent trend is to miniaturize the oxygen-sensitive element of the sensor by constructing it as a laminate of thin, film-like layers on a plate-shaped ceramic substrate of very small size. As described in U.S. Pat. No. 4,224,113, a reference oxygen partial pressure of a nearly constant level can be maintained in this sensor element by supplying a DC current of the order of 10.sup.-6 to 10.sup.-5 A to the concentration cell part of the sensor element so as to flow through the solid electrolyte layer thereby controlling the migration of oxygen ions in the solid electrolyte layer. Since the solid electrolyte does not exhibit its proper function at temperatures below a certain level such as about 400.degree. C., the substrate of the oxygen sensor element is provided with a heater to which an adequate voltage is applied for the purpose of maintaining the sensor element at a nearly constant temperature. However, the temperature of the concentration cell part of the sensor element varies considerably depending on the temperatures of the gases subject to measurement with resultant changes in the level of the reference oxygen partial pressure in the sensor element and in the output characteristic of the sensor element.
In operating the above described oxygen sensor element in automotive engine exhaust gases which undergo frequent changes in the temperature thereof over a very wide range, it becomes a matter of importance to the achievement of accurate detection of oxygen concentrations in the exhaust gases and high-precision control of air/fuel ratio to prevent changes in the level of the reference oxygen partial pressure in the sensor element with variations in the exhaust gas temperature.